Cladding-pumped optical fiber and methods for fabricating

ABSTRACT

Disclosed is an optical fiber article for receiving pump radiation of a first wavelength for amplifying or generating radiation of a second wavelength. The optical fiber article includes a core for propagating light of the second wavelength. The core has a first index of refraction index and includes a rare earth material. A cladding surrounds the core and has a second index of refraction that is less than the first index of refraction. The outer circumference of the cladding can include a plurality of sections, where the plurality of sections includes at least one substantially straight section and one inwardly curved section. The optical fiber article can also include at least one outer layer surrounding the cladding, where the index of refraction of the outer layer is less than the second refractive index. Methods for producing the optical fiber article are also disclosed, as well as methods for providing a preform for drawing such an optical fiber article.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to optical fiber devices and, moreparticularly, to cladding-pumped optical fiber lasers and amplifiers.

2. Background Art

Optical fiber lasers and amplifiers are known in the art. In such lasersand amplifiers, rare earth materials disposed in the core of the opticalfiber laser or amplifier receive pump radiation of a predeterminedwavelength and, responsive thereto, provide or amplify light of adifferent wavelength for propagation in the core. For example, the wellknown erbium doped fiber amplifier (EDFA) receives pump radiation havinga wavelength of 980 or 1480 nanometers (nm) and amplifies an opticalsignal propagating in the core at a wavelength in the 1550 nm region.

In such optical fiber lasers and amplifiers, the pump radiation can beintroduced directly to the core, which can be difficult due to the smallsize of the core, or can be introduced to the cladding surrounding thecore and absorbed by the core as the rays propagating in the claddingintersect the core. Lasers and amplifiers with the pump radiationintroduced to the cladding are known as “cladding-pumped” opticaldevices, and facilitate the scale-up of lasers and amplifiers to higherpower systems.

Absorption per unit length is a useful figure of merit for evaluating acladding-pumped optical fiber laser or amplifier. It is typicallydesirable that the amplifier or laser have a high absorption per unitlength, indicating that the pump radiation frequently intersects thecore. Unfortunately, when the cladding has a circular outercircumference, the pump radiation can essentially propagate down theoptical fiber while spiraling around the core without substantiallyintersecting the core. This leads to a low absorption per unit length ofthe optical fiber device, and hence detracts from the performance of theoptical fiber laser or amplifier.

Various approaches are known in the art for enhancing the intersectionof the pump radiation with the core and hence raising the absorption perunit length of the optical fiber amplifier or laser. For example, asdisclosed in U.S. Pat. No. 4,815,079, issued Mar. 21, 1989 to Snitzer etal., the core can be offset from the center of the optical fiber so asto enhance the intersection of pump light with the core. In anotherapproach, the inner cladding has a “D”-shaped outer circumference thatincludes a flat section, as disclosed in U.S. Pat. No. 5,864,645, issuedJan. 26, 1999 to Zellmer et al. In another prior art optical fiber, theouter circumference of the cladding is shaped as a polygon, such as adiamond, as disclosed in U.S. Pat. No. 5,533,163, issued Jul. 2, 1996 toMuendel. Other approaches include providing a star-shaped outercircumference of the cladding, as disclosed in U.S. Pat. No. 5,949,941,issued Sep. 7, 1999 to DiGiovanri. See also WO 99/30391, published Jun.17, 1999, disclosing an optical fiber having a core, inner and outercladdings, and a series of perturbations or irregularities formed in theotherwise circular outer boundary of the inner cladding. The opticalfiber is drawn from a preform having rods inserted into holes drilledinto the preform for producing the irregularities.

In the foregoing prior art fibers, the non-circular shape of the outercircumference is understood to cause ray distortion and mode mixing oflight, thereby directing the light rays of the cladding radiation to thecore, and avoiding trapping light in spiral paths that do not intersectthe core.

The designs discussed above can have disadvantages. For example, a fiberhaving an offset core can be difficult to interconnect with otheroptical components. Designs, such as the diamond and polygon designsdiscussed above, that require the circumference of the cladding topredominately consist of flat areas, can be difficult to fabricate. Theflat areas, which are typically first machined into the preform fromwhich the optical fiber is drawn, tend to deform and change shape whenthe fiber is drawn at the most desirable temperatures. Accordingly,often the draw temperature is reduced to preserve the desired shape ofthe outer circumference of the cladding. A reduced draw temperaturetypically produces optical fibers having higher attenuation and lowermechanical strength. In addition, the star shaped configurationdisclosed in U.S. Pat. No. 5,949,941 can be difficult to manufacture.Accordingly, an improved cladding-pumped optical device and/ortechniques for manufacturing such optical fiber devices would be awelcome advance in the art.

It is desirable to address one or more of the foregoing disadvantagesand drawbacks of the prior art.

SUMMARY OF THE INVENTION

According to the preferred embodiment, an optical fiber article forreceiving pump radiation of a first wavelength for amplifying orgenerating radiation of a second wavelength includes a core forpropagating light of the second wavelength. The core has a firstrefractive index and includes a rare earth material. A claddingsurrounds the core and has a second refractive index that is lower thanthe first refractive index. The outer circumference of the claddingincludes a plurality of sections, where the plurality of sectionsincludes at least one straight section and one inwardly curved section.An outer layer surrounds the cladding and has an index of refractionthat is less than the second index of refraction.

It is considered that the combination of the straight and inwardlycurved sections in the outer circumference of the cladding enhancesscattering of the pump radiation for more effective absorption of thepump radiation by the core. For example, the inwardly curved section canintercept the pump light reflected from the straight section in asubstantially different direction, thus achieving a higher degree ofrandomization of the paths of the light rays of the pump light forincreased interception of the light by the core of the optical fiberarticle.

Preferably, an optical fiber article in accordance with the inventionincludes four to twelve sections, where each section of the four totwelve sections is one of inwardly curved and substantially straight.Other sections shaped other than straight or inwardly curved may bepresent as well. The inwardly curved and straight sections can alternateabout the circumference of the cladding. Preferably, each of theinwardly curved sections is spaced from the core of the optical fiberarticle, at its point of closest approach to the core of the opticalfiber article, by a distance that is less than or equal to the spacingbetween any one of the straight sections and the core at the point ofclosest approach of any one of the straight sections to the core.

In other aspects of the invention, each of the straight sections isintersected at a substantially perpendicular angle by a different radialvector, and each of the inwardly curved sections are intersected at asubstantially perpendicular angle by a different one of other radialvectors. The different radial vectors are spaced by a first angle, andthe other radial vectors are spaced by a second angle substantiallyequal to the first angle. Preferably, the straight sections are longerthan the inwardly curved sections.

The optical fiber can be adapted for single mode propagation at thesecond wavelength, or alternatively, for propagating a plurality ofmodes at the second wavelength. As is known in the art, in certain fiberdesigns the core and/or the cladding can be characterized by more thanone index of refraction. For example, it is known for the core to have asegmented refractive index profile to broaden the mode fields. Gradedindex fibers are also known. However, fibers having a core and/orcladding characterized by more than one index of refraction are withinthe scope of the invention, because for total internal reflection tofacilitate guiding light in the core, the cladding includes an index ofrefraction that is less than an index of refraction of the core, as iswell known in the art.

The invention can also include methods practiced in accordance with theteachings herein.

In one aspect, the invention provides a method of making an opticalfiber article having an optical fiber core and an optical fiber claddingsurrounding the optical fiber core. The method can include the followingsteps: providing a preform having a preform core and a preform claddingsurrounding the preform core, where the preform core includes a rareearth material and has a selected index of refraction, and the preformcladding has an index of refraction less than the selected index ofrefraction; forming at least one slot in the preform cladding; formingat least one flat area in the preform cladding; and drawing the preformto form the optical fiber article such that the optical fiber articleincludes an optical fiber core surrounded by an optical fiber claddinghaving an index of refraction that is less than the index of refractionof the optical fiber core, and wherein the optical fiber claddingincludes an outer circumference having at least one inwardly curvedsection and at least one straight section. In another aspect of theinvention, the preform can be drawn at a higher temperature moreconducive to providing a lower attenuation and higher strength opticalfiber article.

A glass jacket, having an index of refraction that is less than theindex of refraction of the preform cladding, can be disposed about thepreform cladding and drawn with the preform to provide an optical fiberarticle having a glass outer layer surrounding the cladding. The glassjacket can be collapsed, such as by heating, onto the preform cladding.The outer circumference of the glass jacket can be shaped, such as toreduce the depth of indentations or depressions in the glass jacket.

In another aspect of the invention, glass soot is deposited on thepreform cladding and heated to form a preform outer layer.

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section of an optical fiber article that includesa core and cladding surrounding the core;

FIG. 1B is a cross section of a typical prior art optical fiber having acladding that includes a circular outer circumference;

FIG. 2A depicts a cross section of one embodiment of an optical fiberarticle according to the invention;

FIG. 2B depicts a cross section of another optical fiber articleaccording to the invention;

FIG. 3A schematically illustrates drawing an optical fiber from apreform;

FIG. 3B schematically illustrates drawing an optical fiber from apreform and a glass jacket disposed about the preform cladding;

FIG. 3C schematically illustrates drawing an optical fiber from apreform, wherein the preform includes a glass jacket collapsed about thepreform cladding.

FIG. 4A is a cross section of a circular preform having a preform coreand a preform cladding;

FIG. 4B illustrates flat areas formed in the preform cladding of thepreform of FIG. 4A;

FIG. 4C illustrates slots formed in the preform cladding of the preformof FIG. 4B;

FIG. 4D is a cross section of an optical fiber article where at leastthe core and the cladding are formed from drawing the preform of FIG.4C;

FIG. 5A illustrates collapsing the glass jacket of FIG. 3C about thepreform of FIG. 3C;

FIG. 5B is a cross section of the preform and the glass jacket of FIG.5A, taken along section line 5B—5B of FIG. 5A;

FIG. 5C is a cross section of the preform having the glass jacketcollapsed thereon and taken along section line 5C—5C of FIG. 5A;

FIG. 5D illustrates shaping the preform and glass jacket of FIG. 5C;

FIG. 5E illustrates a cross section of an optical fiber article drawnfrom the preform and glass jacket of FIG. 5D;

FIG. 6A: illustrates depositing glass soot on a preform having a preformcladding that includes slots and flat areas; and

FIG. 6B illustrates a cross section of the preform of FIG. 6A afterheating to sinter the glass soot to form a preform outer layer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a cross sectional view of an optical fiber article 10 thatextends in the longitudinal, or Z, direction, as identified by thecoordinate system 12. The optical fiber article 10 includes a core 14, acladding 16 that surrounds the core 14, and can also include the outerlayer 18 that surrounds the cladding 16. The cladding 16 includes alower index of refraction that the core 14 such that the laser light 22to be amplified or generated is confined largely to the core 14. Theouter layer 18 includes a lower index of refraction than the cladding 16such that the pump radiation 24 is confined largely to the cladding 16.A second outer layer, which can include a plastic or polymer or othersimilar material, can surround the outer layer 18 for protecting theoptical fiber article 10.

The core 14 typically includes one or more rare earth dopants, which canbe selected from the Lanthanide group of elements in the periodic table,in a glass matrix, which can be a silica glass matrix. The silica glassmatrix can include one or more other dopants, such as Ge, P, Al, B, F,etc., and which can be added for a variety of reasons, such as to modifythe refractive index of the core 14 or to improve the performance of therare earth dopants in the core 14. When the pump radiation 24 intersectsthe core 14, the pump radiation 24 is absorbed by the rare earthmaterials, such as erbium, in the core 14 for amplifying or generatingthe laser light 22, which has a different wavelength than the pumpradiation 24. The outer layer 18 cladding can include a low indexpolymer or a low index glass.

FIGS. 2A and 2B are cross sections of optical fiber articles accordingto the invention, and are described below. However, an understanding ofthe problem addressed by the present invention can be furthered by firstconsidering FIG. 1B, which is a cross section of a typical prior artoptical fiber having a cladding 16′ with a circular outer circumference28′. Note that the pump radiation 24′ can spiral around the core 14′without intersecting the core 14′, leading to a low absorption per unitlength of the pump radiation 24′ by the core 14′.

FIG. 2A illustrates a cross section of one embodiment of an opticalfiber article 10 according to the invention. The outer circumference 28of the cladding 16 that surrounds the core 14 includes a plurality ofsections 36 including inwardly curved sections 36A and straight sections36B. It is considered that the combination of straight sections 36B andinwardly curved sections 36A increases the intersection of the core 14by the pump radiation 24 that propagates in the cladding 16.

The inwardly curved section 36A can be located with the straight section36B, along the outer circumference of the cladding, such that theinwardly curved section 36A intercepts pump light reflected from thestraight section 36B in a substantially different direction, thusachieving higher degree of randomization of the path of the light raysof the pump light 24. This leads to increased interception of the pumplight 24 by the core 14 of the optical fiber article 10. For example,ray 40 is shown reflecting off one of the straight sections 36B and thenoff of inwardly curved section 36A for reflection through the core 14.

Shown in FIG. 2A are three (3) straight sections 36B and three (3)inwardly curved sections 36A. However, it is considered that theinvention can be practiced with fewer or more curved and straightsections than are shown in FIG. 2A. Preferably, the straight sections36B and inwardly curved sections 36A jointly number from four to twelvesections. Not all the sections that make up the outer circumference ofthe cladding need be straight or inwardly curved. For example, it may bedesirable to interpose sections having other shapes along thecircumference 28 of the cladding 16. In the embodiment shown in FIG. 2A,the inwardly curved sections 36A alternate about the circumference withthe straight sections 36B.

Though any number of inwardly curved sections 36A and straight sections36B can be used to scatter the pump light 24, and the present inventionis not to be limited to a particular length and curvature of theinwardly curved sections or to a particular length of the straightsections 36B, the following considerations are noted. If the outercircumference 28 includes very few sections, the overall shape of thecircumference 28 can deviate from a circular shape, tending to make theoptical fiber article 10 difficult to cleave and/or splice withconventional circular fibers. Conversely, as the number of sections isincreased, the shape of the circumference 28 tends to become circularand the scattering of the pump light 24, and hence absorption by thecore 14, can be reduced.

Preferably, each of said inwardly curved sections 36A is spaced, at itspoint of closest approach 44 to the core 14 of the optical fiber article10, a distance from the core 14 that is less than or equal to thespacing between any one of the straight sections 36B and the core 14 atthe point of closest approach 48 of that one straight section to thecore 14. The straight sections 36B can be recessed relative to thecurved sections 36A.

FIG. 2B serves to illustrate additional features that can be included inan optical fiber article 10 of the invention. Note that each of thestraight sections 36B can be intersected at a substantiallyperpendicular angle by one of the different radial vectors 52A-52C,which extend from the center 54 of the optical fiber article 10. Theradial vectors 52A-52C are spaced by substantially the same angle,represented by the angle 56 in FIG. 2B. Furthermore, each of theinwardly curved sections 36A can be substantially perpendicularlyintersected by a different one of other radial vectors 58A-58C, and theother radial vectors are spaced by substantially the same angle,represented by the angle 60 in FIG. 2B. In one embodiment of theinvention, the angle 60 is substantially equal to the angle 56. As shownin FIG. 2B, the straight sections 36B are preferably longer than saidinwardly curved sections 36A.

Note that the outer circumference of the cladding 16 can also includeshort sections that are outwardly curved, such as sections 62, typicallyformed during the drawing process described below.

FIG. 3A schematically illustrates drawing an optical fiber article 10Afrom a preform 106A. The preform 106A can be made by one of severalmethods, including vapor phase axial deposition, outside vapordeposition (OVD), or modified chemical vapor deposition (MCVD), as wellas other methods known to those of ordinary skill in the art. A furnace110, such as a high frequency induction furnace or a resistance furnace,heats the preform 106A. A spool 102 pulls the optical fiber article 1OAfrom the preform 106A as the preform is heated by the furnace 110. Adiameter measuring element 114 can be included for monitoring thediameter of the drawn optical fiber article 10A. A coating apparatus 118can be included for providing additional coatings, such as the outercoating 18, over the cladding 16. Apparatus 122 can also be included forcuring the optical fiber article 10A before it is wound on the spool102. A coating or coatings can be added to the fiber article 10A afterthe processing by the apparatus shown in FIG. 3. The arrangement ofapparatus shown in FIG. 3A is exemplary; the term “drawing”, as usedherein, refers to heating glass and pulling a strand of fiber from theglass.

FIGS. 4A-4C illustrate the forming of the preform 106A such that it issuitable for facilitating provision of an outer circumference 28 havinginwardly curved sections 36A and straight sections 36B. FIG. 4A is across section of the substantially circular preform 106A and illustratesthe preform core 132 and the preform cladding 136, from which,respectively, the core 14 and cladding 16 of the optical fiber article10A are formed. The preform core 132 can be doped with rare earth ions(for example, one or more of the Lanthanides, such as Er, Yb, Nd, Tm,Ho, etc.) and other preferred dopants (for example, one or more of Ge,P, Al, F, B, etc.). The preform 106A can be made by conventional methodsknown to those of ordinary skill in the art of making preforms.

As illustrated in FIG. 4B, various substantially flat areas 134 areformed, such as by machining, on the substantially round preform 106A.The substantially flat areas 134 typically extend longitudinally alongthe preform 106A. The substantially flat areas 134 facilitate formationof the straight sections 36B of the optical fiber article 10A upondrawing of the preform. For example, in the particular case shown inFIG. 4B, flat areas 134 that are disposed at 90 degrees from each otherare machined in the preform 106A. Note the distance “d” of the flat area134 to the center of the preform 106A can be varied. If R is the radiusof the glass preform, the dimension d is preferably as follows:0.7R≦d≦0.97R. More preferably, d is as follows: 0.8R≦d≦0.96R. Mostpreferably, d is given by:

0.9R≦d≦0.95R.

As illustrated in FIG. 4C, slots 140, preferably rectangular in shape,can be formed, such as by machining, in the preform 106A. The slots 140are typically formed between each pair of adjacent flat areas 134 andtypically extend longitudinally along the preform 106A. The width “W” ofthe slot 140 and depth “h” of the slot 140 can be varied to givedifferent shape of the resultant inwardly curved section 36A of theouter circumference 28 of the optical fiber article 10A. The selectionof the dimensions W and h each can affect the curvature of the resultantinwardly curved section 36A of the circumference 28 of the cladding 16of the optical fiber article 10A. The value of h preferably is asfollows: 0.5(R−d)≦h≦2.5(R−d). More preferably, h is as follows:0.75(R−d)≦h≦2.4(R−d). Most preferably, h is defined by1.0(R−d)≦h≦1.5(R−d). For a given length L of the flat areas 134 formedor to be formed on the preform 106A, W is preferably as follows:0.1L≦W≦0.9L. More preferably, W is defined as follows: 0.2L≦W≦0.7L. Mostpreferably, W is defined as follows: 0.3L≦W≦0.6L.

FIG. 4D is a cross section of the optical fiber article 10A having acore 14 and cladding 16 formed from drawing the preform of FIG. 4C. Theslots 140 in the preform 106 facilitate the formation of inwardly curvedsection 36A in the outer circumference 28 of the cladding 16 and theflat areas 134 of the preform facilitate the formation of the straightsections 36B in the outer circumference 28 of the cladding 16. Note thatthe optical fiber article 1OA in FIG. 4D includes four inwardly curvedsections 36A and four straight sections 36B, and both the inwardlycurved sections and the straight sections are, as is also shown in FIGS.2A and 2B, substantially equally spaced about the outer circumference ofthe cladding 16. An outer layer 18 and a second outer layer 19 are shownin FIG. 4D, where one or both of the layers can be, for example, apolymer or plastic layer. One or both can be added prior to or afterspooling of the drawn optical fiber article 10A.

Typically, the outer layer 18 includes a polymer layer selected suchthat the index of refraction of the layer 18 is lower than the index ofrefraction of the cladding 16. The second outer layer 19 can be anacrylic polymer or other polymer layer that is included for protectingthe optical fiber article. Both can be added by a suitable coatingapparatus 118, which can include chambers or coating cups, etc., as isknown in the art.

Thus, according to the invention, there can be provided an improvedoptical fiber wherein the outer circumference of the cladding isselectively shaped. Prior art shaped fibers, such as those discussed inthe Background Art section above, are typically drawn at temperaturessubstantially lower than those used when drawing standard round fiber.These reduced temperatures can be required to preserve the desired shapeof the outer circumference of the cladding of the resultant drawn fiber.In the prior art processes, it is desired that the shape of the crosssection of the preform becomes the shape of the outer circumference ofthe cladding of the resultant optical fiber. Drawing at the highertemperature tends to round the straight areas in the outer circumferenceof the cladding of the fibers, and can change the angle between thesections, and hence, according to the prior art, is often avoided.Unfortunately, drawing a fiber at reduced temperatures can havedisadvantages, as the fibers tend to have higher light attenuation andare physically weaker than those drawn at higher temperatures. Thus,prior art fibers require a compromise.

In practicing the invention, a higher draw temperature can be used, andthe rounding effect advantageously used to promote desired shapes of theouter circumference 28 of the cladding 16 of the optical fiber article10, such as the formation of the inwardly curved surfaces 36A. The useof a higher temperature aids in achieving better fiber strength andlower attenuation. Furthermore, the combination of the inwardly curvedsections 34A and straight sections 34B is understood to enhance theintersection of the pump light 24 with the core 14.

The draw temperature is preferably selected to be high enough to allowflow and reshaping of the preform when drawn such that the slots 140flow to become inwardly curved.

FIG. 3B illustrates an alternative approach for adding the outer layer18. A jacket 150 is disposed about the preform 106B and is drawn withthe preform 106B using the furnace 110. The jacket 150 is typically acylinder of glass, and can include a fluorinated or borosilicate glass.The outer layer 18 is thus formed on the cladding 16 from thefluorinated or borosilicate glass. A vacuum is drawn as indicated byreference numeral 152, on the space 154 between the glass jacket 150 andthe preform 106B. In the approach shown in FIG. 3B, the coatingapparatus 118 can be used to add the outer layer 19, which can be abuffer layer, over the outer layer 18. An outer layer 18 that includes aglass is considered advantageous due to difficulties associated withpolymer outer layers 18 that have the desired index of refraction lowerthan the index of refraction of the cladding layer 16. For example,fluorinated polymers can be inferior in terms of mechanical strength,permeability to moisture, and long-term reliability. However, the outercircumference 156 of layer 18 can include depressions or indentationsformed where the outer layer 18 conforms to the slots 140 of the preformor to the inwardly curved sections 36A of the cladding 16. See, forexample, reference numeral 158 in FIG. 4D.

In another approach, shown in FIG. 3C, the jacket 150 is collapsed ontothe preform 106C prior to drawing of the optical fiber article 10C. Asshown in FIG. 3C, the space 154 between the jacket 150 and the preformcladding 136 is reduced or eliminated.

FIG. 5A illustrates collapsing the jacket 150 of FIG. 3C about thepreform 106C of FIG. 3C. As indicated by reference numeral 152, a vacuumis drawn on the space between 154 between the preform 106C and thejacket 150, as a heat source 160, in this instance a flame, heats thejacket 150. The heat source 160 can be moved along the jacket 150, asindicated by reference numeral 162 to more evenly heat the jacket 150,thereby collapsing the jacket 150 onto the preform cladding 136, andadding the jacket 150 as a third layer to the preform 106C, as indicatedby reference numeral 164. Typically the preform 106C and jacket 150 arerotated for evenly distributing the heat from the flame.

FIG. 5B is a cross section of the preform 106C and the jacket 150 ofFIG. 5A, taken along section line 5B—5B of FIG. 5A, and FIG. 5C is across section of the preform 106C having the jacket 150 collapsedthereon and taken along section line 5C—5C of FIG. 5A. Note that theslots 140 have now become rounded, as indicated by reference numeral140′, forming inwardly curved sections in the outer circumference 168 ofthe preform cladding 136. Some rounding may also occur in the flat areas134 of the preform cladding 136. It is also possible that the jacket150, when collapsed as shown in FIG. 5C, includes depressions orindentations 170 that correspond to the location of the slots 140 in thepreform cladding, such that the outer circumference 172 of the jacket150 deviates from being circular. A fiber drawn from the preform 106Cand jacket 150 can also therefore include an outer layer 18, formed fromthe jacket 150, that includes an outer circumference that deviates fromcircular, and includes the depressions and indentations 158 shown inFIG. 4D.

It is preferred that the outer circumference of the outer layer 18 of anoptical fiber article 10 is characterized by a single diameter forfacilitating mating of the optical fiber article 10 with other opticalfibers or components. Accordingly, FIG. 5D illustrates shaping thepreform 106C of FIG. 5C so as to reduce the depth of indentations ordepressions 170 in the outer circumference thereof formed during thestep of collapsing the jacket 150. Typically, the preform 106C is shapedvia machining with a machine tool 176 of a lathe as the preform 106C isrotated, as indicated by reference numeral 180, such that the outercircumference 180 of the jacket 150 becomes substantially circular. Thepreform can also be shaped via grinding, such as by centerless grindingtechniques, or by other shaping techniques understood by those ofordinary skill to be appropriate. FIG. 5E illustrates a cross section ofthe optical fiber article 10C drawn from the preform 106C including thejacket 150 of FIG. 5D. The second outer layer 19 is added, such as bythe coating apparatus 118 in FIG. 3C. In the embodiment shown in FIG.5E, the outer circumference 185 of the outer layer 18 is substantiallycircular, and can be characterized by single diameter when specifyingconnecting the optical fiber article 10 to other optical fibers orcomponents.

With reference to FIG. 6A, in yet another approach, glass soot 200 isdeposited on the preform cladding layer 136. The glass soot 200 can beproduced by a flame 204 fed by a fuel 206 and a suitable chemical vapor210. The chemical vapor can be silicon tetrachloride mixed with afluorine bearing material or boron bearing material, such as BCl₃, BBr₃,SiF₄, or SF₆. The preform cladding layer 136 can be rotated, asindicated by reference numeral 215, to promote even distribution of theglass soot 200. The glass soot 200 can then be heated to sinter the sootand to form a preform 106D having the preform core 132, the preformcladding 136, and a preform outer layer 220, as shown in FIG. 6B,showing the preform in cross section. The foregoing method can produce aglass preform 106D having an outer layer 220 that is more circular, orat least having depressions or indentations of reduced depth, such thatthe shaping operation, such as is shown in FIG. 5D, may be avoided or,if the preform 106D is shaped, less material of the outer layer 220 willrequire removal. The preform 106D can then be drawn, generally as shownin FIG. 3A, into an optical fiber article having a core, cladding andouter layer. The resultant optical fiber article would generally appearas shown in FIG. 5E.

It will thus be seen that the invention efficiently achieves the objectsset forth above, as well as those apparent from the foregoingdisclosure. It is intended that all matter included in the abovedisclosure be interpreted as illustrative and not in a limiting sense,as one of ordinary skill in the art, apprised of the disclosure herein,can make certain changes in the above constructions without departingfrom the scope of the invention. For example, sections other thanstraight sections and inwardly curved sections can be deliberatelyincluded in the outer circumference of the cladding, and the straightsections need not necessarily be tangential to a circle about the center54 of the optical fiber article, as shown in FIGS. 2A and 2B. As anotherexample, the jacket 150 need not be limited to glass, but can includeother materials understood to be suitable by one of ordinary skill inthe art, apprised of the disclosure herein.

Accordingly, it is understood that the following claims are intended tocover generic and specific features of the invention described herein,and all statements of the scope of the invention which as a matter oflanguage might be said to fall therebetween.

We claim:
 1. An optical fiber article for receiving pump radiation of afirst wavelength for amplifying or providing light of a secondwavelength, comprising: a core for propagating light of a secondwavelength, said cow having a first refractive index and including arare earth material; a cladding surrounding said core and having asecond refractive index that is lower than said first refractive index,said cladding including an outer circumference having a plurality ofsections, said plurality of sections including a plurality of inwardlycurved sections and a plurality of substantially straight sections, saidinwardly curved sections and said straight sections alternating alongthe outer circumference of said cladding; and an outer layer surroundingsaid cladding, said outer layer having an index of refraction that isless than said second index of refraction.
 2. The optical fiber articleof claim 1 wherein said outer layer includes a glass.
 3. The opticalfiber article of claim 2 wherein said outer layer includes an outercircumference that is substantially circular.
 4. The optical fiberarticle of claim 2 wherein said outer layer includes an outercircumference that is substantially free of a depression disposed oversaid at least one inwardly curved section.
 5. The optical fiber articleof claim 1 wherein said plurality of sections includes four to twelvesections, each section of said four to twelve sections being one ofinwardly curved and straight.
 6. The optical fiber article of claim 1wherein said plurality of sections includes a plurality of substantiallystraight sections and a plurality of inwardly curved sections, andwherein each of said straight sections is adjacent to inwardly curvedsections and wherein each of said inwardly curved sections is adjacentto straight sections.
 7. The optical fiber article of claim 1 whereinsaid plurality of sections includes a plurality of substantiallystraight sections and a plurality of inwardly curved sections, andwherein each of said inwardly curved sections is spaced from said coreof said optical fiber article, at its point of closest approach to saidcore of said optical fiber article, by a distance that is less than orequal to the spacing between any one of said straight sections and saidcore at the point of closest approach of said any one of said straightsections to said core.
 8. The optical fiber article of claim 1 whereinsaid plurality of sections includes a plurality of said substantiallystraight sections and a plurality of said inwardly curved sections, andwherein each of said straight sections is intersected at a substantiallyperpendicular angle by a different radial vector, said different radialvectors being spaced by a first angle, and wherein said inwardly curvedsections are each substantially perpendicularly intersected by adifferent one of other radial vectors, said other radial vectors beingspaced by a second angle substantially equal to said first angle.
 9. Theoptical fiber article of claim 1 wherein each of said straight sectionsof said at least one straight section is longer than each of saidinwardly curved section of said at least one inwardly curved section.10. The optical fiber article of claim 1 wherein said optical fiber isadapted to propagate a single mode of light having the secondwavelength.
 11. The optical fiber article of claim 1 wherein saidoptical fiber is adapted to propagate a plurality of modes of lighthaving the second wavelength.
 12. The optical fiber article of claim 1wherein said core includes a silica glass.
 13. The optical fiber articleof claim 1 wherein said at least one inwardly curved section is formedfrom a slot in a preform from which said core and said cladding aredrawn.